In some high voltage power supply applications, an AC-to-DC converter is used to convert an AC input voltage, such as an AC line voltage from a power outlet, to a regulated DC output voltage at a desired output voltage level. To meet regulatory requirements, AC-to-DC converters are often designed with power factor correction (PFC) to achieve a high power factor while reducing total harmonic distortion. Various topologies exist for AC-to-DC converters incorporating power factor correction (PFC). For example, a conventional boost topology uses a bridge rectifier (also referred to as a diode bridge) to rectify the AC input voltage to DC followed by a boost converter functioning as an active PFC circuit.
FIG. 1 is a schematic diagram illustrating a part of the circuitry of a conventional AC-to-DC power converter. Referring to FIG. 1, a power converter 10 receives an AC input voltage VAC 12 and generates one or more regulated DC output voltages, typically at voltage levels lower than the AC input voltage. The power converter 10 includes a diode bridge 14 for converting the AC input voltage VAC 12 to a fully rectified DC voltage VDC. The fully rectified DC voltage VDC is coupled to a power factor correction (PFC) circuit which is implemented as a boost converter in the present example. The PFC boost converter may include an isolation transformer with the primary winding 16 driven by a switching transistor Q1, usually a MOSFET transistor. The switching transistor Q1 is being turned on and off under the control of a control circuit, such as PWM control circuit 24. The source terminal of NMOS transistor Q1 is connected to the ground potential (node 22) while the drain terminal is connected to the primary winding 16 (node 18).
In order for the AC-to-DC power converter 10 to operate, the PWM control circuit 24 needs a source of power (voltage Vdd) to turn on so as to drive the switching transistor Q1. The power supply voltage Vdd for the PWM control circuit 24 is typically generated from the same AC input voltage VAC. The power converter 10 typically includes a start-up circuit to supply power to the PWM control circuit 24 as the AC power is being turned on. For example, it is known to use a normally on device, such as a JFET, an ACCUFET, or a Depletion mode MOSFET, at the DC voltage node to charge up the power supply voltage Vdd so that the control circuit can turn on to start the power converter operation. In the present example, a JFET J1 is coupled between the primary winding (node 18) and a capacitor C1 (node 20). The capacitor C1 provides the power supply voltage Vdd to the PWM control circuit 24. As soon as the AC voltage VAC is turned on, JFET J1 is conducting and the capacitor C1 is charged up to provide PWM control circuit 24 with the power supply voltage Vdd. When the voltage at capacitor C1 reaches a threshold level, the PWM control circuit 24 turns on to control switching transistor Q1. The AC-to-DC power converter then operates to generate the power supply voltage Vdd for the PWM control circuit 24. Once the power supply voltage Vdd (node 20) reaches the threshold level, the JFET J1 is turned off. In some embodiment, the JFET J1 is an N-channel device and the threshold voltage to shut off the JFET is a gate-to-source voltage VGS of about −5V.